This application relates to a system for dispensing a desired amount of a viscous fluid at a dispensing point. More particularly, the disclosed invention provides accurate metering of the quantity of viscous fluid dispensed.
Viscous fluids are generally difficult to pump or dispense. Rotary pumps may not adequately move a viscous fluid, and thus reciprocating pumps are typically utilized. As is known, the use of a reciprocating pump results in pulses in the fluid flow downstream of the pump. When the reciprocating pump is charging, or receiving fluid to be pumped in its chamber, no fluid is being discharged and there is a substantial reduction in the rate of fluid flow. These low points in the flow rate cycle typically alternate with high flow pulses which occur as the pump does discharge. In order to reduce the duration and extent of these low flow rate points, double-acting reciprocating pumps are sometimes utilized. In such pumps, opposed faces of a piston are typically driven to empty pump chambers 180.degree. out of phase from each other such that the time period between discharges of a pump chamber is reduced. The use of the two pump chambers attempts to align the discharge of one pump chamber with the low flow rate portion of the cycle which occurs during filling of the other pump chamber. Even with double-acting pumps, however, there is a period of time between the discharging of the two chambers in which neither chamber is discharging. This time delay causes undesirable irregularities and low flow rates in the total fluid flow. If such periodic low flow rates occur at a dispensing point, it becomes difficult to dispense a desired amount of fluid, or to accurately meter the rate and amount of fluid dispensed, since there may not be uniform flow of fluid at the dispensing point.
Also, in order to move highly viscous fluids it becomes necessary for the pump to generate very large pressures in the fluid. Tubing which may connect the pump to the distribution point may expand due to the high fluid pressure. This expansion can allow the fluid pressure to drop, causing further problems in supplying a desired amount of fluid to the distribution point.
Further, it is difficult to accurately meter viscous fluids from a reciprocating metering valve, such as a needle valve. High pressure, high viscosity fluids may resist movement of the reciprocating valve member, thus making it difficult to accurately and rapidly control the amount of fluid dispensed.
The above-discussed problems exist in dispensing viscous adhesives, and in particular adhesives used to secure glass members to a vehicle body. Urethane adhesives, typically utilized for this purpose, are non-Newtonian fluids that are highly viscous. One such urethane adhesive has a viscosity of 6,000,000 centipoise. Urethane adhesives typically include an isocynate pre-polymer and a catalyst. One particular adhesive which utilizes water as the catalyst, is formulated to begin curing immediately upon leaving the distribution nozzle, when it is exposed to water vapor in ambient air.
Urethane adhesives are expensive and thus it is desirable to reduce the amount utilized to the minimum required to achieve the desired results. The amount of adhesive needed often varies with location on the members to be joined. As an example, it may be necessary to have a greater amount of adhesive at the sides or bottom of a vehicle windshield than is needed at the top of the windshield. For this reason it is desirable that the adhesive be accurately metered when dispensed onto a member to avoid waste.
In known systems, a double-acting reciprocating pump is mounted on top of a cylinder containing the urethane adhesive. In order to supply the viscous fluid to the inlet of the pump, an arrangement known as a pressure primer is utilized. A pressure primer includes a plate forced downwardly into the cylinder to move the urethane adhesive up to the inlet of the pump. As discussed above, the double reciprocating pump used to move the adhesive to the dispensing point typically results in pulses in the flow. In response to this problem, one prior art device has utilized a "shot-meter" system to eliminate any short-term deficiency of adhesive caused by the low flow rate portions of the cycle. Shot-meter systems as used in this prior art system include a pair of large cylinders that are alternately communicated to the reciprocating pump such that one cylinder is always receiving adhesive from the reciprocating pump. The second cylinder is typically being driven to discharge adhesive that had previously been stored and force it to the dispensing point.
In such shot-meter systems, the cylinder receiving fluid may have a piston floating upwardly until the piston contacts a switch. The switch reverses valves such that the cylinder which was previously discharging begins to receive fluid from the pump, while the other cylinder forces fluid to a dispensing point.
While shot-meters do eliminate pulses in fluid flow, they are complex, heavy, require large amounts of floor space and are expensive. Some prior art systems have utilized nozzles mounted to a robotic manipulator. One such system suggests positioning a shot-meter system on a robot arm, so that the relatively pulseless flow out of the shot-meter system will reach the nozzle prior to travelling through long lengths of tubing. In practice, however, the size of the shot-meter system may make it impractical to position it as close to the nozzle as desired.
Further, shot-meter systems include several surface areas that must be tightly sealed. The adhesives typically utilized to secure vehicle glass cannot be exposed to air while in the shot-meter system or they will begin to cure. This increases the complexity of the shot-meter system.
Also, the shot-meter system typically stores fluid in one cylinder while discharging fluid from the other cylinder. All fluid dispensed is stored for at least a temporary period of time in a cylinder. This is undesirable, at least with the particular adhesive identified above, since the adhesive begins to cure upon contact with air. Even though a shot-meter system is designed to be air-tight it is still desirable to move the adhesive to a dispensing point as quickly as possible, and not leave it stored in any intermediate member for any unnecessary length of time since there could be air leakage. Further, the requirement that all of the adhesive to be dispensed be stored in a cylinder increases the size of the shot-meter cylinders.
In the known system mentioned above, a reciprocating tapered pin is used in the metering valve. As discussed above, reciprocating pins are not capable of accurately and rapidly metering viscous fluids. The metering valve is attached to an arm of a robotic manipulator which moves the nozzle about a first member to dispense adhesive on the first member. In this system, a reciprocating pump and pressure primer arrangement is communicated to a shot-meter system, which is in turn communicated to the reciprocating metering nozzle mounted on a robot arm. The overall arrangement is large and requires a great deal of floor space in an assembly area. In particular, the shot-meter system requires a large amount of floor space. In designing a system to be utilized in modern assembly environments it is preferred that a minimum floor space be utilized.
For the above reasons, it is an object of the present invention to provide a system and method for accurately and rapidly metering a desired amount of viscous fluid to a dispensing point. More particularly, it is an object of the present invention to provide such a system and method utilized to meter a desired amount of a viscous adhesive onto a first member which is to be attached to a second member.